<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box{
            width: 100%;
            height: 500px;
            border: 3px solid gray;
            display: flex;
            /*align-items: flex-start;*/
            /*align-items: flex-end;*/
            align-items: center;
            /*align-items: baseline;*/
            /* align-items: stretch; */
            justify-content: space-evenly;
        }
        .box div{
            width: 300px;
            color: #ffffff;
            text-align: center;
        }
        .box1{
            height: 200px;
            background-color: #ff0000;
        }
        .box2{
            height: 200px;
            background-color: #bdff60;
            font-size: 30px;
        }
        .box3 {
            height: 400px;
            background-color: #7570ff;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="box1">第一个盒子</div>
    <div class="box2">第二个盒子</div>
    <div class="box3">第三个盒子</div>
</div>
</body>
</html>